This Non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s). 093111988 filed in Taiwan, Republic of China on Apr. 29, 2004, the entire contents of which are hereby incorporated by reference.
The present invention relates to a broadband monopole antenna, and in particular to an antenna with omnidirectional horizontal radiation.
For higher wireless transmission rates, a broadband antenna is popularly used, especially those capable of providing omnidirectional radiation. Recently, planar metal-plate monopole antennas are frequently used. Although frequency ratios over 1:3 are available, omnidirectional radiation characteristics (e.g. radiation pattern) thereof are unstable, generally decreasing with operating frequency. This behavior is mainly because, for achieving a very wide impedance bandwidth, the planar metal-plate monopole is usually designed to be with a large width, which is usually comparable to or larger than a quarter-wavelength of the highest operating frequency in the impedance bandwidth. In this case, due to the path-length difference caused by the large monopole width, the antenna's radiated fields contributed from the excited surface currents near the two side edges of the planar monopole will be destructive in the direction parallel to the planar monopole. On the other hand, the radiated fields in the direction normal to the planar monopole in general have no path-length difference and will be constructive. This behavior will lead to poor omnidirectional radiation characteristics for the conventional planar monopole antenna.
U.S. Pat. No. 4,466,003 discloses a conventional monopole antenna comprising several metal rods of various lengths creating various resonance frequencies. However, this monopole antenna is oversized and presents a complicated configuration.
A broadband monopole antenna disclosed in U.S. Pat. No. 5,828,340 increases 40% of the operation bandwidth, but cannot satisfy commercial requirement.
U.S. Pat. No. 6,339,409 discloses a helical antenna formed by a right-angle triangular conductive plate. Although wider bandwidth is available, this antenna presents an even more complicated structure.